Structures deployable in space, of the solar generator type for example, are generally made up of rigid panels articulated to one another, these panels when in the stored position being stacked on top of one another. These structures have the advantage of having dynamic behaviour that is fully controlled but have the disadvantage of having a high inertia and mass per unit area. Furthermore, rigid structures when in a stored position occupy a significant amount of space under the cap of a launcher. Because the amount of space allocated to deployable structures under the cap of a launcher is limited, it is important to reduce the space occupied by these deployable structures when they are in the stored position, so as to optimize the surface area they represent in the deployed position.
There are deployable flexible planar structures that comprise a flexible fabric and tape springs fixed to one and the same plane of the fabric. In the stored position, the fabric and the tape springs are wound around a mandrel. Deployment of the flexible planar structure is brought about autonomously by the spontaneous unwinding of the tape springs when the mandrel is free to rotate.
Indeed tape springs are known in the field of space as being flexible tapes with a circular arc-shaped cross section, the radius of curvature of which circular arc is convex on a first face and concave on a second face, these tapes being able to pass from the wound state to the unwound state essentially through their own elastic energy. There are various types of tape that have their own properties. Monostable tapes have a deployed natural position and need to be held in the stored position. Monostable tape springs therefore have a natural tendency to deploy in order to regain their unwound state. The deployment of monostable tapes is often disorganized and uncontrolled. Bistable tapes have two natural positions (stored position and deployed position) and do not need to be held in the stored position when the cross section is fully flattened. Their deployment is linear and controlled. However, in both instances, when deployment is initiated it may be violent and generate shocks, which means to say that the entire tape spring may have a tendency to straighten out simultaneously, over its entire length, presenting a problem of damage to surrounding elements or elements fixed to the tape spring such as a flexible membrane, an instrument, an antenna, etc. Conventional tape springs may thus present difficulties in terms of controlling their deployment. In order to regulate the speed of deployment of this type of structure, there are a number of methods that can be used. Mention may for example be made of regulation using an electric geared motor unit as described in patent application FR12/03300 or thermal regulation using hybrid tape springs as described in patents FR 0803986 and U.S. Pat. No. 7,856,735.
Furthermore, the stiffness of the tape springs varies according to the axis of stressing. A force F applied to the convex face of the tape spring will have a tendency to cause the tape spring to flex, whereas the same force applied to the concave face will have no effect, and this presents a problem of instability of the flexible structure in its deployed state. In order to address this problem of stability in the deployed state, it is therefore necessary for the tape spring to be kept in the deployed position by an additional retaining means or for the tape spring to be over-engineered so that it remains stable under the orbital forces, whatever the direction in which these are applied.
Thus, in the stored configuration, the tape spring needs to be as compact as possible, which means to say that it needs to have the smallest possible radius of winding. This parameter is given by the physical characteristics of the tape; in general the radius of winding is substantially equal to that of its radius of curvature. In the case of a composite tape, it may be altered by changing the order of stacking of the plies and/or the direction of the fibres. In the deployed configuration, the best possible rigidity is sought, which means the largest and most closed cross section possible, combined with the end of the tape spring being built in as far as possible.